# std::swap(std::optional)

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< cpp‎ | utility‎ | optional

Defined in header `<optional>` |  |
template< class T > void swap( std::optional<T>& lhs, std::optional<T>& rhs ) noexcept(/* see below */); |  | (since C++17) (constexpr since C++20)

Overloads the std::swap algorithm for std::optional. Exchanges the state of lhs with that of rhs. Effectively calls lhs.swap(rhs).

This overload participates in overload resolution only if std::is_move_constructible_v<T> and std::is_swappable_v<T> are both true.

### Parameters

lhs, rhs | - | `optional` objects whose states to swap

### Return value

(none)

### Exceptions

`noexcept` specification:
noexcept(noexcept(lhs.swap(rhs)))

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_optional` | `202106L` | (C++20) (DR20) | Fully constexpr

### Example

Run this code

    #include <iostream>
    #include <optional>
    #include <string>

    int main()
    {
        std::optional<std::string> a{"██████"}, b{"▒▒▒▒▒▒"};

        auto print = [&](auto const& s)
        {
            std::cout << s << "\t"
                         "a = " << a.value_or("(null)") << "  "
                         "b = " << b.value_or("(null)") << '\n';
        };

        print("Initially:");
        std::swap(a, b);
        print("swap(a, b):");
        a.reset();
        print("\n""a.reset():");
        std::swap(a, b);
        print("swap(a, b):");
    }

Output:

    Initially:   a = ██████  b = ▒▒▒▒▒▒
    swap(a, b):  a = ▒▒▒▒▒▒  b = ██████

    a.reset():   a = (null)  b = ██████
    swap(a, b):  a = ██████  b = (null)

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
P2231R1 | C++20 | `swap` was not constexpr while the required operations can be constexpr in C++20 | made constexpr

### See also

swap | exchanges the contents (public member function)

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